For now, the NREL will merely monitor the drive-cycle characteristics of the existing buses before making recommendations regarding the conversion process, according to Green Car Congress (via Charged EVs).

The propane buses have been in service since 2000, when the National Park Service instituted a shuttle operation to reduce congestion on Zion's main roads.

Now the buses are due for replacement, and officials are considering a conversion from propane to electric power.

It previously facilitated the replacement of gasoline and diesel vehicles in parks with low- and zero-emission alternatives, including propane-powered trucks and vans, electric cars, and plug-in hybrids.

Nearly 60 percent of vehicles owned by the National Park Service are electric or alternative fuel, according to the NREL.